This invention relates to a combustion or burning method of a fuel-air mixture in a gas turbine combustor suitable for use in a gas turbine power generation system, and more specifically to a nitrogen oxides decreasing combustion method which features a low level of occurrence of nitrogen oxides (hereinafter called "NO.sub.x ") during combustion and good combustion efficiency and can thus be suitably applied to the catalytic combustion system.
Reflecting recent depletion of petroleum resources and other energy resources, there is a demand for various alternative energy. At the same time, there is also a standing demand for more efficient utilization of energy resources. As means capable of satisfying such demands, there are for example the gas turbine/steam turbine combined cycle power generation system and the integrated coal gasification gas turbine system/steam turbine combined cycle power generation system. Since these power generation systems enjoy higher power generation efficiency compared with the conventional power generation system relying upon steam turbines, they are expected to find commercial utility as power generation systems capable of effectively converting fuels such as natural gas and coal gasified gas, whose production is expected to increase in the future, to electric power.
For gas turbine combustors employed in the gas turbine power generation system, there has been adopted the homogeneous reaction system combustion method in which a mixture of a fuel and air is ignited by means of a spark plug or the like. One example of such combustors is illustrated in FIG. 1. In the combustor shown in FIG. 1, a fuel injected through a fuel nozzle is mixed with burning air (i.e., air for combustion) 3 and is then ignited by a spark plug 2 to undergo its combustion. The resulting gas, namely, the combustion gas is added with cooling air 4 and diluent air 5 to lower its temperature to predetermined gas turbine inlet temperature. Thereafter, the thus-cooled and diluted combustion gas is injected through a turbine nozzle 6 into a gas turbine. In the figure, numeral 8 indicates a swirler.
One of the most serious problems which the above-exemplified conventional combustor is accompanied with is that a great deal of NO.sub.x is produced upon combustion of the fuel, whereby to induce environmental pollution and the like. This occurrence of NO.sub.x is attributed to the development of a localized high-temperature zone, the temperature of which exceeds 2,000.degree. C., in the combustor during the combustion of the fuel.
A variety of combustion methods have been studied with a view toward overcoming such a problem. The heterogeneous reaction system method making use of a solid catalyst (hereinafter called "the catalytic combustion method") has been proposed recently.
In this catalytic combustion method, a mixture of a fuel and air is caused to burn using a catalyst. According to this method, the combustion may be triggered at a relatively low temperature. This method does not require cooling or diluting air and allows to increase the amount of burning air. Thus, the catalytic combustion method has lowered the highest temperature and has hence made it possible to reduce the occurrence of NO.sub.x to an extremely low level.
FIG. 2 is a schematic illustration of one example of combustors which may be used in accordance with the above-described catalytic combustion method. In this figure, the reference numerals identify like elements of structure in FIG. 1. This combustor is equipped, as its structural feature, with a catalyst-packed zone 7.
In this catalyst-packed zone 7, a honeycomb structured catalyst for combustion is usually packed. A mixture of a fuel and air is brought into contact with the packed catalyst there, thus causing the mixture to burn through a catalytic reaction.
An exemplary temperature distribution of a gas stream and packed catalyst in a combustor, to which the catalytic combustion method has been applied, is illustrated in FIG. 3 in relation to the direction of the gas stream.
In FIG. 3, the zone A-B corresponds to a zone in which a fuel and air are mixed. A mixture, which has been formed there owing to the mixing of the fuel and air, is then brought into a packed catalyst in a catalyst-packed zone corresponding to the zones B-C and C-D. In the zone B-C, the mixture undergoes a catalytic reaction only on the surface of catalyst. Thus, the temperature of the catalyst rises like the zone B'-C' indicated by a dashed line. As a result, the temperature of the gas stream in the catalyst-packed zone also goes up. In the zone C-D, the reaction rate is increased further on the catalyst because the temperature of the stream has already increased in the zone B-C. Therefore, the temperature of the catalyst rises like the zone C'-D' indicated by a dashed line. As a result, the temperature of the catalyst becomes higher than the ignition temperature of the gas stream present in the catalyst-packed zone and the gas-phase combustion (i.e., the homogeneous reaction) also occurs in this zone. Namely, both catalytic reaction and gas-phase combustion take place simultaneously in the zone C-D. This is a typical feature of the catalytic combustion method. Finally, the stream flown out of the zone C-D travels toward the inlet of the turbine while allowing any un-burnt portion of the fuel to burn in its gas phase. This travelling takes place in the zone D-E.
It is disclosed in Japapnese Patent Publication No. 36294/1977 and U.S. Pat. Nos. 3,914,090; 3,928,961; 3,940,923; 3,982,879; 4,019,316 and 4,065,917 to the effect that in a combustion method as mentioned above, the temperature of the packed catalyst ranges from 815.degree. to 1650.degree. C. in the catalyst-packed zone corresponding to the zones B-C and C-D.
However, the above-proposed method is also accompanied by a problem that the temperature of the packed catalyst is required to reach a relatively high temperature in the zone C-D, in other words, the temperature of the catalyst packed in the zone C-D has to be higher than the ignition temperature of the gas stream which is brought into contact with the catalyst. When a fuel difficult to undergo gas-phase combustion, such as methane gas, is employed for example, the fuel is unable to substantially burn up unless the temperature of the catalyst is 1000.degree. C. or higher. Therefore, a catalyst to be packed there is required to successfully withstand temperature above 1000.degree. C. or preferably 1100.degree. C.
Under the circumstances, no one has however succeeded to develop a catalyst capable of withstanding such high temperature to permit its utilization over a long period of time under such severe temperature conditions. Accordingly, it is extremely difficult to practice such a combustion method as illustrated in FIG. 3.